This invention relates to an internal resistive (or conductive) coating for a cathode ray tube and a method of making the coating. More particularly, the invention relates to an internal coating derived from a suspension consisting essentially of conductive and insulative particles, silicate solids, water and a wetting agent of the anionic class.
It is a common practice in present-day cathode ray tube manufacture to provide an internal resistive coating intermediate an anode button on the funnel portion and a snubber affixed to a mount assembly sealed into the neck portion of a cathode ray tube envelope. This internal resistive coating is employed to dissipate undesired arc currents which frequently occur due to the relatively high potentials and closely spaced components within the envelope. Thus, dissipation of undesired arc currents at the source or within the cathode ray tube tends to inhibit the undesired application thereof to associated semi-conductor components which are prone to catastrophic failure when subjected to such stresses.
In addition to this arc-limiting capability, there is a similar group of coatings having conductive and insulative particles therein which are used primarily for the purpose of increased abrasion resistance. The abrasion resistance characteristic is dependent upon the hardness of the insulator particles and the solids concentration of the silicates. Thus, improved abrasion resistance in the vicinity of the gun snubbers and near an antenna getter in the funnel of the cathode ray tube tends to reduce the number of abraded particles and the number of arcs occurring within an operating tube.
Normally, this group of coatings has a smaller proportion of insulative particles and lower electrical resistance than the previously mentioned "arc limiting" resistive coatings and is sometimes referred to as internal conductive coatings. However, for the purposes of this disclosure, the term resistive coatings refers to both the lower electrical resistance and the "arc limiting" type coatings.
An example of a commonly used internal resistance coating is one wherein iron oxide, graphite, a silicate solution and a dispersing agent are combined to provide a resistive coating. This coating is frequently applied to the inner surface of the cathode ray tube intermediate an anode button and a mount assembly snubber by a "brush" technique. In other words, a brush is dipped into the coating and applied to the open funnel while the open funnel is rotated prior to panel to funnel frit sealing, and prior to mount assembly sealing and exhaust or "finished" processing into a cathode ray tube.
Although the above-described technique of applying a resistive coating by a "brush" has been and still is employed with varying degrees of success, it has been found that the resultant cathode ray tube structure still has numerous deficiencies and problems. More specifically, it has been found that the capabilities for providing a uniform resistive valve for a multiplicity of cathode ray tube leaves something to be desired when the above coating and "brush" technique is utilized.
As mentioned above, the known resistive coating formulations include graphite particles therein. It has been found that at least some of the graphite particles have a tendency not to become "wetted" whereupon a graphite "screen" or "insulator-poor" portion of coating results. Moreover, that portion of the coating immediately adjacent the "insulator-poor" portion will obviously be characterized as an "insulator-rich" portion of coating.
While the above-mentioned formulations may or may not include a dispersing agent therein, it is known that a dispersing agent tends to dissolve in the aqueous solution. Thus, the dispersing agent has little or no effect upon the "wetting" of the graphite particles.
Further, it is also known that dispersing agents tend to provide an action which is electro-kinetic in nature whereby a negative charge is imported to a particle causing the particles to repel one another and thus provide the dispersing feature. Moreover, a poly-molecular layer or film is also often created when the dispersing agent is absorbed by the particle and this layer or film acts as a physical barrier to direct contact between the particle and other particles or surfaces.
Thus, a process for applying such a suspension by a "brush" technique would obviously encounter the problem of non-uniformity of the applied suspension. As a result the non-uniformity of the suspension would produce a non-uniform resistive coating with non-uniform resistive values.
Further, dispersing agents tend to form a film on the particles rather than to "wet" the particles of the suspension. In turn, the particles do not have the capability to "wet" the surface whereon the suspension is applied. Because of this lack of "wetting" action, the suspension tends to be deposited unevenly leaving undesired brush marks. Moreover, the undesired brush marks in the applied suspension dry into undesired high resistive rings in the resultant resistive coating.